1. Field of the Invention
The present invention relates to a magnetic-resonance imaging diagnosis apparatus and a magnetic-resonance imaging method.
2. Description of the Related Art
Conventionally, according to a magnetic-resonance imaging diagnosis apparatus, a subject placed on a top-plate mat on a couch is fit with a Radio Frequency (RF) coil for local imaging as required, and inserted into the inside of a magnet that includes an opening, and then imaging to take a magnetic resonance image is performed. Specifically, the magnetic-resonance imaging diagnosis apparatus reconstructs an magnetic resonance image by radiating an RF magnetic field onto the subject placed in static magnetic field and collecting Magnetic Resonance (MR) signals emitted from the subject owing to the RF magnetic field, based on a certain data-collecting method (for example, a spin echo method, or a fast spin echo method).
If the subject moves while imaging, an artifact due to the motion is produced on a magnetic resonance image. Such artifact causes difficulty in imaging diagnosis by a doctor with the use of a magnetic resonance image. For this reason, generally an operator of a magnetic-resonance imaging diagnosis apparatus orally tells a subject before imaging not to move during the imaging.
However, despite that imaging is started after orally telling the subject not to move; when determining that it is to take only a magnetic resonance image unavailable for imaging diagnosis because the subject moves during the imaging, the operator discontinues the imaging and then performs a retake under the same conditions. Particularly, when imaging a subject who has difficulty in keeping a steady position due to an advanced age or a disease, a possibility of a retake is high.
To avoid retake, imaging to take a magnetic resonance image is performed by a data collecting method by which motion of a subject can be corrected.
For example, a magnetic resonance image on which motion of a subject is corrected can be reconstructed by using data collected by a method of periodically rotated overlapping parallel lines with enhanced reconstruction (PROPELLER method) (for example, see Isao Muro, et al., “Effect of Motion Correction Associated with Echo Train Length and Number of Blades in PROPELLER MRI-Computer Simulation”, Japanese Journal of Radiological Technology, Vol. 60, No. 2, pp. 264-269).
As shown in FIG. 10, the PROPELLER method is a method of collecting k-space data in a frequency region in a non-orthogonal manner by rotating a belt region called “blade”, which is formed of a plurality of parallel data collection traces, every repetition time. The PROPELLER method is also called a BLADE method, and hereinafter simply referred to as a “blade-rotating data-collection method”, and a pulse sequence according to the “blade-rotating data-collection method” is hereinafter referred to as a “blade-rotating data-collection sequence”.
As shown in FIG. 10, according to data collected by the “blade-rotating data-collection sequence”, data in the vicinity of the center of the k-space (at which the frequency is “0” in the phase encoding direction and the frequency encoding direction) is present in each blade without exception. Accordingly, by comparing images obtained from data on the k-space through a Fourier transform performed blade by blade, a shift amount between the images corresponding to respective blades in different time sequences can be determined. FIG. 10 is a schematic diagram for explaining the blade-rotating data-collection method.
Based on the determined shift amount, deviations between the images are corrected by relative rotational shift and parallel shift, and a Fourier transform is again performed on k-space data created through an inverse Fourier transform from images corresponding to respective corrected blades, so that a magnetic resonance image can be reconstructed on which influence of motion is suppressed, i.e., artifacts caused by motion of a subject are suppressed.
According to the conventional “blade-rotating data-collection method” described above, because k-space data in a frequency region is filled by overlapping, an imaging time tends to be long. In other words, if imaging is performed by the “blade-rotating data-collection method” from the beginning in order to avoid difficulty in an imaging diagnosis by a doctor, an imaging time for each individual subject tends to be long, resulting in a problem that the imaging diagnosis becomes inefficient.